If you wish to only subscribe for one month, you can do so. For that, you will need to cancel the subscription before the billing date is due. If you cancel after the billing date has passed, you will be charged for a second month of subscription. Read more about How to cancel your subscription.
The subscription is, by definition an auto-renewing plan. Depending on what billing you choose for your subscription, it will renew every 30 days (for monthly billing) or every 12 months (for annual billing).
Once your subscription is active, it will stay active until you cancel it. If you do not wish to use the subscription for more than 1 month, you are free to cancel it at any given time before the billing period has ended. If you cancel your subscription after the billing period expired, it will be automatically renewed and you will be charged for a second month.
In order to cancel your subscription , you need to go to Your account and click on the Cancel Subscription button on that page.
Whenever you need icons again, you can always reopen the subscription from Your account page.